1. Field of the Invention
This invention relates to computer memory interfaces, digital computers and electronic data processing systems.
2. Description of Prior Art
Digital processor systems ordinarily include a central processing unit with a memory. In microcomputer applications the program memory is contained on the same chip as the central processing unit. However, in larger microcomputer systems the program memory may be implemented on an external memory system set of chips and interfaced over an input/output line to the microcomputer chip containing the central processing unit. In digital processing systems requiring external memory, the interface between the memory chip and the central processing unit chip may be accomplished with either a hand shaking technique, i.e., the CPU chip sends a request over I/O line, then answered by the memory chip over the same I/O line before the transfer was actually implemented, or the transfer is synchronous, that is, the central processing unit request memory access and then at a certain set time, read the data that is available on the input/output lines from the external memory system. Using a synchronous interface to an external memory saves time and improves the performance of the microcomputer system. However, the information from the external memory must be available on the input/output data lines when the central processing unit chip expects to read the data. Problems arise when different external memory systems are used with the same microcomputer chip set since the external memory systems may and most probably do have different access time requirements. If the external memory system is faster than is required by the microprocessor system, then there is no problem if the external memory system can hold the data on the input/output lines until read. However, if the external memory system is slower than the microprocessor or microcomputer system, then errors may occur when the microcomputer or system attempts to read the data on the input/output bus since there is no guarantee that the data on the bus at the time it is read is the data that is being accessed from the external memory system. This invention solves the foregoing problem by providing an external memory interface for a microcomputer system that provides an automatic wait time to allow slower external memory systems to interface to the microcomputer system.